The present invention concerns cellulose materials. By "Cellulose material" there are understood either plant materials containing both cellulose and other products, for example lignin, or cellulose obtained by any process, in particular processes based on plant materials.
The invention concerns in particular an anisotropic composition having a base of at least one cellulose derivative comprising ester groups of cellulose, at least a part of these ester groups being formate groups. The invention also concerns articles obtained from this composition, the cellulose formate of which articles can be regenerated to cellulose. The invention also concerns processes which make it possible to obtain these compositions and articles, as well as articles of high mechanical properties, these articles having a base of cellulose and/or at least one cellulose derivative as previously defined. Such articles may, for instance, be fibers or films.
German Pat. Nos. 189 836, 189 837 and 561 119, French Pat. Nos. 638 431 and 541 048, British Pat. Nos. 260 650 and 275 641 and U.S. Pat. Nos. 1,656,119 and 1,900,599 describe processes for transforming cellulose into cellulose formate by the action of formic acid in the presence of various inorganic compounds which may, in particular, by hydrochloric acid, sulfuric acid or phosphoric anhydride. The processes described in said patents lead to at least one of the following drawbacks:
impossibility of obtaining a solution of cellulose formate;
extensive degradation of the cellulose upon the esterification reaction or storage, particularly when the temperature becomes greater than 0.degree. C., the cellulose formate having a degree of polymerization which is definitely less than that of cellulose before esterification;
impossibility of obtaining articles of high mechanical properties.
U.S. Pat. No. 2,363,091 describes the production of mixed formate-acetate esters of cellulose by the action of mixed formic-acetic anhydride on cellulose in the presence of a catalyst, which may, for instance, be phosphoric acid or phosphoric anhydride, the amount of this catalyst not exceeding 30% of the weight of the cellulose. The process described in said patent does not make it possible to obtain dissolution of the cellulose.
German Pat. No. 227 198 describes the dissolving of cellulose by the action of acetic acid and phosphoric acid. This patent does not mention the production of a solution of cellulose formate. Furthermore, the process described does not make it possible to obtain cellulose acetate concentrations sufficient to obtain anisotropic solutions.
British Pat. No. 263 810 describes the dissolving of cellulose by the action of phosphoric acid and an organic acid, which may be acetic acid, propionic acid or butyric acid. This patent does not describe the production of a solution of cellulose formate. Furthermore, the concentration of the cellulose esters in the solutions described in said patent is not sufficient to obtain anisotropic solutions.
British Pat. Nos. 309 (1911) and 29246 (1910) as well as Swiss Pat. No. 56146 and German Pat. No. 249535 describe processes in which cellulose is contacted with a mixture of concentrated formic acid and phosphoric acid. The examples described in these patents show that a cellulose solution is not obtained or that an isotropic solution is obtained.
German patent application OS No. 3 035 084 describes the production of anisotropic solutions by dissolving cellulose esters in aqueous solutionsof inorganic acids. This process does not apply to cellulose formate. Moreover, this process is expensive since it uses cellulose esters and not cellulose as raw material. Furthermore, these cellulose esters, in general, have a substantially lower degree of polymerization than the cellulose used to obtain them. It may be added that the mechanical properties of the articles described in said application and produced from these esters are low. Finally, this application does not describe any regeneration of the cellulose from the cellulose ester solutions; moreover, such a regeneration would be very slow with the solutions described and practically impossible to carry out continuously.